In the past, two-way radio transceivers transmitted and received speech signals in analog form. An audio speech signal produced by a microphone at the transmitter was amplified and processed by analog circuits, and applied to the RF transmitter for "modulating" an RF carrier signal. The RF carrier carrying this analog audio signal was transmitted over the air and received by the receiver. The receiver "demodulated" the received RF signal to recover the analog audio signal, which it then amplified and applied to a loudspeaker. In this way, a person at the receiver could hear the words spoken by the person at the transmitter.
More and more throughout the communications industry, digital signal processing techniques are replacing analog techniques. Modern two-way radio transceivers employ significant digital signal processing capabilities, and perform on digital signals many of the processes that used to be performed in the analog domain. FIG. 1 is a schematic illustration of an example of some of the digital signal processing performed by a modern digital two-way radio.
FIG. 1 shows two radio transceivers 50A, 50B. Each of these transceivers 50A, 50B can transmit or receive. In FIG. 1, transceiver 50A on the left is shown in the transmit mode, and transceiver 50B on the right is shown operating in the receive mode. Thus, speech spoken at the transmitter 50A is carried on a radio wave W over the air to the receiver 50B where it can be heard by someone at the receiver end.
To transmit signals over the air, the user speaks into a microphone 52 at transmitting radio 50A. Microphone 52 converts the user's speech sounds into an analog audio signal. This analog audio signal is applied to a digital conversion process 54 that converts the analog audio signal into digital signals. The resulting digital signals are then "compressed" by a compression process 56. The purpose of this "compression" is to decrease the overall "data rate" of the digital signals. By squeezing the digital signals into a smaller "bandwidth," radio 50 can achieve higher speech fidelity (frequency range) over the narrow bandwidth of a radio channel.
The compressed digital signals are then applied to an encryption process 58. Encryption process 58 uses a special mathematical transformation known as "encryption" (also known as "enciphering") to convert the digital signals into a form that is unintelligible to anyone who does not know the special inverse "decryption" transformation needed to transform the signals back into their original ("clear" or "plain text") form. Encryption is used to ensure secrecy of the communications by preventing eavesdroppers intercepting the communications without authorization from understanding the communications. The output of encryption process 58 is applied to a modulation process 60 which "modulates" a RF carrier with the encrypted digital data. The RF carrier is applied to antenna 62a for radiation over the air.
Radio 50B in the receive mode receives the transmitted RF signal on its antenna 62b and "demodulates" the RF carrier to recover the original digital signal generated by encryption process 58 within the transmitting radio 50A. Demodulation process 64 is the inverse of modulation process 60. This recovered encrypted digital signal is applied to a decrypting process 66. Decrypting process 66 performs the inverse of the encryption process 58, and thus transforms the encrypted received digital signals back into unencrypted ("plain text") digital signals. These "clear" digital signals are run through an "expanding" process 68 which performs an inverse of the compression process 56. The resulting decompressed digital signals are applied to the input of an analog conversion process 70 that performs the inverse of digital conversion process 54 within transmitting radio 50A, i.e., it converts the digital signals back into analog signals. These analog signals are amplified and applied to a loudspeaker 72. The loudspeaker 72 converts the analog signals into sound waves so that a person at the receiving can hear the same sounds spoken by the person at the transmitting end.
Prior to this invention, digital radios manufactured and sold by Ericsson-GE Mobile Radio Communications Inc. (the assignee of this patent) used separate integrated circuit chips to perform the "codec" processes 54, 70;the compression/decompression processes 56, 68; and the encrypting/decrypting processes 58, 66. For example, Ericsson-GE's prior MPA, MPD and AEGIS digital two-way radio products used commercially-available chips called "codecs" (coder and decoder) to perform the analog-to-digital and digital-to-analog conversion processes 54, 70. These prior products used a separate programmed digital signal processor (DSP) chip to perform the "vocoding" processes 56, 68, compressing the digital signal on the transmit end and expanding the digital signal on the receive end. Commercially-available encryption/decryption ASIC chips were used in these products to encrypt and decrypt the digital signals per processes 56, 66.
In these products, a radio control processor (a further microprocessor chip) was used to coordinate operations between the codec, the vocoder DSP, and the encryptor/decryptor ASIC chip. This additional microprocessor chip (which typically also provided all of the high level control functions for the entire radio) moved speech data between the codec chip and the vocoding DSP chip; between the vocoding DSP chip and the encryptor/decryptor chip; and between the encryptor/decryptor chip and a transceiver modem. These data transfers were asynchronous, and the control microprocessor was required to reformat the data to at each of these transfer stages match the requirements of the codec chip, the vocoding DSP chip, and the encryptor/decryptor chip. These real time functions imposed significant timing constraints on the control microprocessor, since it required several different data protocols, a data translation process, and timing-sensitive implementation. In addition, this 3-chip architecture (in addition to the control microprocessor) had the disadvantage of increasing the cost of the radio and limiting its flexibility.
To solve these problems, a mixed signal, digital signal processor had been designed to incorporate the codec, vocoding and encryption processes in a single chip. Briefly, in accordance with the preferred embodiment of the present invention, a single digital signal processor module (DSP) performs the "vocoding" process (speech compression and decompression) and the encryption/decryption process in an integrated manner in real time. The preferred embodiment employs a DSP chip including a built-in analog-to-digital converter and digital-to-analog converter, so that the "codec" process can also be integrated into the same speech processing module. The speech processing module provided by the present invention thus performs any/all of analog/digital conversion, speech vocoding, and encryption/decryption in real time without need for intervention by the radio control processor. The speech processing module of the preferred embodiment isolates the radio control microprocessor from the data-handoff between vocode and encrypt operations. This results in less data shuffling and fewer commands. Therefore, the control microprocessor no longer requires digital speech processing to occur in high priority interrupts. In addition, only one new protocol is needed between the radio control microprocessor and the speech processing module.
Some of the features and advantages provided by the present invention include:
Codec, compression and encryption functions are all combined within a single digital signal processor integrated circuit chip. PA1 Compression and encryption are performed in a real time integrated manner without need for external intervention by the control processor. PA1 Protocol between the speech processing module and the radio control processor provides for efficient data transfers without undue loading of radio control processor. PA1 Internal executive routine within speech processing module handles vocoder/encryption commands and data processing. PA1 User-defined encryption feature. PA1 Modem/codec synchronization handled automatically. PA1 DES encryption is directly integrated in software with a radio product. PA1 Fewer chips. PA1 Less control processor loading. PA1 Increased flexibility. PA1 Radio control processor can control speech processing module to operate as stand-alone encryptor/decryptor. PA1 Real time speech processing with selectable vocode only, encrypt only, or vocode and encrypt.